Birth characteristics of children with congenital dislocation of the hip (CDH) were compared with a control group of births matched with cases by date of birth, sex, race, mother's residence at time of birth, and father's occupation. Cases were predominantly Caucasian with a marked seasonal variation in the distribution of births. The largest proportion of cases was bom in the coldest months, suggesting an environmental influence present throughout the year but most active during cold months. Cases delivered by breech did not exhibit this seasonal trend, although breech deliveries were recorded 4 times as frequently for cases as for controls. A larger than expected proportion of cases were first births and cases weighed, on the average, 0.16 pounds less than controls. These differences were eliminated when breech cases were analyzed separately. Low birth weight and low birth order appeared to be only secondarily associated with CDH by virtue of their strong primary association with breech delivery. The strong association of breech delivery and CDH a unexplainable. Factors operative in the causation of CDH may also be operative in producing the breech presentation. At all birth ranks except 3, mothers of cases were younger than controls with the same birth rank.